The present invention relates to capacitors used in various electronic equipment, particularly to an anode member using valve metals for a solid electrolytic capacitor, an electrolytic capacitor using the same, and a method of making the same.
Generally, electrolytic capacitors used for e.g. secondary circuits of power sources and CPU peripheries of personal computers are strongly desired to be smaller in size and higher in capacitance, and also to be lower in the ESR (equivalent series resistance) for adaptation to high frequencies.
FIG. 1 is a perspective view of an anode member for a conventional solid electrolytic capacitor of such kind. Referring to FIG. 1, reference numeral 1 is a lead wire for an anode terminal, reference numeral 2 is an anode member made by molding a valve metal powder of e.g. tantalum or niobium to a given shape and by embedding the lead wire 1 in the mold of the vale metal powder. This kind of structure is called pellet structure, which is generally often used.
More specifically, the anode member 2 is made by: molding the valve metal powder having the lead wire 1 embedded therein to a molded body of the given shape; sintering the molded body; subjecting the sintered body to anodic oxidation to form a dielectric oxide film (not shown) on its outer surface; and then sequentially forming a solid electrolyte layer (not shown) on the dielectric oxide film and forming a cathode layer (not shown) thereon. An anode terminal (not shown) for external connection is connected to the lead wire 1 of the anode member 2, and a cathode terminal (not shown) for external connection is connected to the cathode layer. The thus processed body is subjected to a molding using a molding material such as epoxy resin to coat the anode member 2 with the molding material, thereby to make a solid electrolytic capacitor.
Furthermore, recently solid electrolytic capacitors using, as electrolyte, electrically conductive polymers, which have incomparably higher electric conductivities than conventionally used manganese dioxide, were developed and commercialized in order to realize lower ESR.
However, such a conventional anode member and an electrolytic capacitor using the anode member as described above have drawbacks in the difficulty of realizing smaller capacitors with higher capacitance.
A specific example of prior attempt to make such conventional electrolytic capacitors smaller in size and higher in capacitance is disclosed in Japanese Laid-open Patent Publication 2000-306782. FIG. 2 is a perspective view, partially in cross section, of a single layer solid electrolytic capacitor according to such prior attempt. As shown in FIG. 2, a solid electrolytic capacitor is proposed, which has: an anode body 3 made of a valve metal foil; a sheet form anode layer 4 formed on the anode body 3 and made of a sintered body having micro pores and being of a high CV valve metal powder; and a solid electrolyte layer 5 and a cathode layer 6 formed on the sheet form anode layer 4.
The above described anode body and the anode layer, as well as inner surfaces of the micro pores in the anode layer, have a dielectric oxide film formed thereon by having been subjected to anodic oxidation. However, on the surface of the valve metal foil of the anode body 3, impurities such as Fe, Si, alkaline metals and alkaline earth metals are likely to segregate. Furthermore, the surface area of the valve metal foil is incomparably larger than that of a lead wire having a usually often used diameter of 0.25 mm. Accordingly, such structure as of FIG. 2 is comparatively more influenced by the impurities in the metal foil composition, whereby leakage current of the resultant electrolytic capacitor increases. In other words, because of the use of the valve metal foil for the anode body 3, the dielectric oxide film formed by the anodic oxidation are likely to have more defects, so that the resultant solid electrolytic capacitor is likely to suffer larger leakage current.
It is an object of the present invention to solve the problems of such prior art, and to provide an anode member for a solid electrolytic capacitor which, by using a high CV powder, can realize smaller size, higher capacitance, smaller leakage current and lower ESR. The present invention also provides an electrolytic capacitor using the anode member, and a method of making the anode member.
An anode member according to the present invention has a structure, comprising an anode body made of a valve metal foil having a purity of not less than 99 wt % and an anode layer made of a sintered body of a valve metal powder, which anode layer is formed on the anode body. By suppressing the amount of the impurity contained in the valve metal foil to an amount not more than 1 wt %, impurities in a dielectric oxide film formed by anodic oxidation can be lowered, so that defects in the dielectric oxide film can be decreased. Therefore, leakage current of the resultant solid electrolytic capacitor can be decreased.
The anode body preferably has a rough surface, i.e. alternating convex-concave surface. Thereby, impurities segregated on the surface of the valve metal foil can be lowered. At the same time, metal bonding between the anode body of the valve metal foil and the anode layer of the valve metal powder sintered body can be improved. Consequently, leakage current of the resultant solid electrolytic capacitor can be decreased.
Furthermore, it is preferred that tantalum or niobium be used for the valve metal foil, and that tantalum or niobium each of a purity of not less than 99 wt % be used for the valve metal powder in the valve metal powder sintered body. Thereby, good forming or anodic oxidation property, namely homogeneous composition and uniform thickness of the dielectric oxide film, can be realized with the impurity contained in the valve metal powder sintered body being suppressed to an amount of not more than 1 wt %, resulting in decreased defects in the dielectric oxide film. Accordingly, the resultant solid electrolytic capacitor can enjoy a decreased leakage current and a higher capacitance.
The solid electrolytic capacitor according to the present invention comprises the following four elements that are sequentially stacked: an anode body made of a valve metal foil having a purity of not less than 99 wt %; an anode layer made of a valve metal powder sintered body; a solid electrolyte layer; and a cathode layer. Thereby, a solid electrolytic capacitor having a small leakage current can be obtained with a stable reproducibility.
Still further, the solid electrolyte layer preferably comprises an electrically conductive polymer material. Thereby, the electrical conductivity of the solid electrolyte layer can be made very high, and thus a very low ESR can be realized.
Another structure of a solid electrolytic capacitor according to the present invention comprises a stack of at least two solid electrolytic capacitor elements stacked on each other, each solid electrolytic capacitor element comprising an anode body made of a valve metal foil having a purity of not less than 99 wt %, an anode layer made of a valve metal powder sintered body, a solid electrolyte layer, and a cathode layer that are sequentially stacked on each other. Thereby, a solid electrolytic capacitor of a still lower ESR and a still higher capacitance can be obtained.
A method of making an anode member for a solid electrolytic capacitor according to the present invention comprises a step of forming a rough surface on an anode body by subjecting the surface of the anode body made of a valve metal foil having a purity of not less than 99 wt % to an etching treatment or a blasting treatment, and a step of forming, on the anode body, an anode layer made of a sintered body of a valve metal powder. By this method, a solid electrolytic capacitor having a small leakage current can be easily obtained.
While the novel features of the present invention are set forth particularly in the appended claims, the present invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.